1. Field of the Invention
The present invention relates to imaging systems. More specifically, the present invention relates to imaging spectrometers.
2. Description of the Related Art
Military seekers have evolved and become increasingly more complex. Twenty-five years ago, seekers were only concerned with the intensity of a target. Today there are imaging seekers which take into consideration the total image of the target, as well as its spectral signature. Current one-color imaging systems do not provide enough information to make unique identification of items within the field of regard. These imagers can provide the spectral signature of a target as a whole, but not the spectral distribution of each and every pixel in the image.
Many attempts to field a multi-spectral imager have been made, some more successful than others. One successful prior art imager is the computer tomographic imaging spectrometer (CTIS). Developed in the mid 90's, the CTIS does not utilize spatial scanning such as commonly found in pushbroom types of instruments, nor does it employ interferometric scanning such as found in Fourier-transform spectrometers which are employed as imaging devices by utilizing a focal plane array (FPA) in the focal plane. The CTIS achieves true flash spectral imaging by aperture-coding spectral and spatial information onto the FPA using a computer-generated holographic (CGH) disperser. Restoration of the measured data is necessarily performed after the data is collected so that there is a required time delay between capture of the (xf, yf) focal plane data, and generation of the true (x, y, λ) object cube.
While the CTIS solved the problem of providing imagery in many spectral bands, it had several limitations. First, it demonstrated inefficient use of the focal plane, providing images in small numbers of pixels. A 1024×1024 pixel focal plane yielded only an 80×80 pixel image in 108 color bands. Second, the CTIS had poor signal to noise ratio (SNR) caused by the design which required all diffraction orders to not overlap thus presenting low intensity to each pixel. Third, it had high instrument self-radiance caused by the CTIS design which employed an intermediate focal plane with collimation stage. Furthermore, the CTIS showed very poor performance in the infrared spectrum, and the mechanical layout resulted in a device which was too large for many applications. Plus, the technology could not be retrofit into existing sensor packages.
Hence, a need exists in the art for an improved system or method for obtaining multi-spectral image measurements which offers better infrared performance than the prior art and is of size and cost suitable for use in military seeker applications.